1. Field of the Invention
The present invention relates to electrical optical displays, especially for displays which are based on the principal of the twisted nematic (TN) liquid crystal cell, and in particular, to a nematic liquid crystal display (LCD) device which provides improved contrast in a wide range of viewing conditions.
2. Description of the Prior Art
To begin, it would be beneficial to briefly review the construction and operation of a typical twisted nematic (TN) LCD. Heretofore, field effect LCD cells, such as those of the TN type, have been constructed typically as follows. Referring to FIG. 1, a liquid crystal material 10 having a positive dielectric anisotropy is interposed between an upper 12 and a lower 14 parallel glass substrate, with the molecules of the liquid crystal material parallel with the upper and lower substrates and twisted 90 degrees therebetween. Such an LCD cell is disposed between a pair of polarizers (16 and 18) with polarizing axis intersecting with each other at right angles. With this typical construction, light impinging upon a device from light source 15 is first polarized linearly by one of the polarizers 18, then its polarized plane is rotated 90 degrees by the liquid crystal's molecules in a twisted arrangement, and finally, light transmits through the other polarizer 16 to be observed by viewer 20. Where transparent electrodes 19 formed with a pattern of a letter, digit, or other symbols are disposed on the inner surface of the upper and lower substrates, and are impressed with a voltage greater than the threshold voltage of the device, the liquid crystal molecules will be arranged in substantially the vertical direction or in the direction of the field. Under these conditions, the polarized plane of incident light will not be rotated by the liquid crystal molecules. Thus, the incident light will be intercepted by the second polarizer 16 creating a dark symbol on a light background. The pattern thus can be displayed by controlling the direction of the liquid crystal medium within the LCD cell to yield the rotation or non-rotation of the plane of polarization.
While this type of arrangement has worked well in sunlight, in darkness, the typical LCD device has been difficult to see due to losses from surface reflection and transmission coefficients. To solve this problem in darkness, typical LCD devices are illuminated with additional light to permit the viewer to see the display. This can be accomplished by flooding the LCD with light from the front or back. However, lighting the LCD under dim or dark conditions requires additional electronics and increases current usage to illuminate the display.
Continuing with our example of the typical TN LCD, in some cases, only light from the viewer's side is available. Referring to FIG. 2, there is shown a side view of a TN LCD device of FIG. 1. In order to achieve a good contrast in a LCD device operated by reflection, a reflector 22 with high reflective capacity and also with suitable diffuse scattering power is situated beneath the lower polarizer to reflect the light from the viewer side back to the viewer's eye, 20.
Having discussed the basic operating properties of a TN LCD, it is important to briefly describe its uses. With the advance of technical innovation, the use of LCDs has grown. LCDs have significant advantages over light emitting diodes in that they essentially act as a capacitor by holding the impressed voltage (i.e. field effect) and consume almost no power. For this reason, LCDs are being used in watches and as readouts for hand held computers, electronic games or small portable devices
In the design of electronic equipment, the general intent has been to reduce the overall size of the equipment. This is particularly true in the selective call radio paging receiver market where recently designed "pagers" are now available that can be carried in a shirt pocket. Such a design typically includes a rectangular parallel piped plastic housing that includes a single printed circuit board with appropriately attached electronic components. The housing is typically not much wider than the thickness of the printed circuit board and components. Therefore, if a visual display is to be installed on a pager, a low profile assembly is usually required. To achieve this low profile assembly the additional lighting to illuminate the LCD under dim conditions requires considerable mechanical design work, power drain, and hence increased costs.
Although older paging receiver designs convey the received message to the user mainly upon receipt, recently developed paging receiver designs store a received message in the pager for later retrieval by the user Therefore, it becomes necessary to inform the user that a message has been received This is typically accomplished by sounding an alert tone, flashing light, actuating a vibrator, or in the case of a digital display pager, causing a predetermined symbol to appear on the paging receiver's display. If a visual indication of the message is desired, the current drain of the light source becomes critical because paging receivers typically have very limited battery capacity Therefore, to reduce battery drain while maintaining the output light intensity at an acceptable level, it would be desirable to provide a visual display that minimizes the amount of current drain from the battery while maintaining adequate legibility of the message on the display under all viewing conditions.
To assure adequate legibility, there must be adequate visual contrast between the alphanumeric characters that comprise the message and the background against which they are viewed.